1. Field of the Invention
The present invention relates to an apparatus and method for filtering particulates of various sizes from miscellaneous process liquids, and more particularly to an apparatus and method that utilizes a filtration bed formed from particles having a specific gravity lower than that of the liquid being filtered.
2. Description of the Prior Art
A preliminary patentability and novelty search regarding the invention described herein has revealed the existence of the following U.S. Pat. Nos.:
3,067,358
3,469,057
3,678,240
3,962,557
4,387,286
4,417,962
4,839,488
4,952,767
5,434,381
5,386,094
A careful review of the patents noted above has failed to reveal the concept, apparatus and method disclosed herein.
The need to remove particulates, whether contaminants or products, from process liquids is common to a wide range of processes. One such process is the necessity to filter metal particles from the solution used during electrical discharge machining. In the following description, the focus will be on the removal of particulate contaminants from such a solution, however, the same mechanisms can be applied to the filtration and harvesting of particulate materials which form the product(s) of a process. Although a variety of methods have been developed to remove particulates from such process liquids, the most popular method is media filtration. In media filtration, particulate contaminants are strained from the process liquid in one of two ways, either by pumping the contaminated liquid through a unitary permeable element, or by pumping the liquid through a filter bed which is itself composed of small particles.
In permeable unitary element filtration, the liquid is pumped through an element which has pores or channels that allow the liquid to pass through the element but prevent the passage of particulates larger than the pore/channel diameter. Permeable elements comprise a variety of materials, including fabric, paper, ceramic, metal and plastic. These elements filter the liquid primarily by capturing the contaminant particles on the surface of the element, thus building up a crust or layer of contaminants on the surface. As contaminants accumulate on the surface of the element, liquid flow through the permeable element is reduced because the crust or layer of contaminants acts as an obstruction and because an increasing number of the pores or channels become blocked. As the percentage of blocked pores or channels increases and the crust or layer of contaminants becomes thicker, the pressure required to maintain a specific rate of flow of liquid through the permeable element increases. Eventually, the pressure required exceeds the capability of the pump, or some other system component, and the contaminated element must be replaced with a new element in order to maintain the desired performance of the filtration system.
Alternatively, an attempt may be made to clean the filter element by backwashing it with clean liquid or air to remove the contaminant accumulated on the surface. However, even when the contaminant accumulation on the surface of such an element is removed by backwashing, there are usually contamination particles that remain lodged in the permeable element that backwashing is not totally successful in removing. Ultimately, the element must either be replaced with a new element or cleaned in a more rigorous fashion, i.e., by immersion in an acid or base solution to dissolve the contaminants. The more frequently such stringent cleaning or element replacement must be performed, the more costly this filtration process becomes.
In contrast, the second type of media filtration, namely, bed filtration, uses a filter bed composed of small particles such as sand or diatomaceous earth, and is one of the most common conventional methods of removing particulate contaminants from liquids. The sand filter uses sand particles that are about 0.35 mm in diameter and fairly uniform in size. Diatomaceous earth filters use a siliceous material formed from the skeletons of small (about 100 microns in diameter) marine algal cells called diatoms. Nominally, in a conventional bed filter, the process fluid is pumped, or allowed to flow via gravity, downward through a column, or bed, of material. This column may range from approximately one foot to several feet in thickness. As the particulate-laden liquid passes through the bed, the particulates are strained from the liquid and the cleaned liquid exits at the bottom of the bed.
The bed filter removes the particulate contaminants via one of two processes. First, the larger particulates, which are unable to pass through the spaces between the bed grains, are trapped at the top surface of the bed. This straining effect produces a layer, or crust (also called a cake), of large contaminant particles, which builds up on the surface of the bed, a mechanism called surface filtration. This cake can actually enhance the performance of the filter bed by helping to capture more contaminant particulates, which are retained in the crust itself because they cannot pass through the spaces between the contaminant particles which form the crust.
Second, smaller particulates which are carried into the bed by the fluid flow are intercepted by the bed""s grains as they follow the convoluted flow pathways taken by fluid as it passes through the bed, a process called depth filtration. Although smaller particulates are captured in the bed material, the smallest particulates are not captured, as they continue to flow through the bed and exit with the semi-cleaned liquids at the bottom of the filter bed.
Ultimately, the particulates sequestered by the bed accumulate, making it more difficult for liquid to flow downward through the bed, and thus the flow rate declines. The pressure required to force liquid through the bed then increases, and presents an excellent indication of the growing need to cleanse the bed of the accumulated particulates. Cleansing is achieved by a process of backwashing or backflushing.
During backwashing, clean fluid is vigorously pumped upwards from the bottom of the particulate bed. This upflow of liquid causes the bed to expand slightly, freeing the captured particulates and washing them upwards and out of the bed. As the bed expands, the bed particles have less interference with each other and thus settle faster, matching the upflow rate of the liquid. This effect prevents the bed particles from being washed out of the bed along with the contaminant particulates. Typical backwash conditions are five to fifteen minutes duration with the bed volume expanded 15 to 30%.
Although sand and diatomaceous earth filters have been successfully applied to a wide variety of filtration problems, they have a number of limitations and drawbacks. For example, one of the most serious problems involves bed homogeneity. Non-homogeneous beds, for example, develop cracks that offer regions of less flow resistance in the bed. These cracks then typically enlarge and lead to the formation of channels in the bed, in turn causing poor distribution of the liquid flow through the bed, and thus very low particulate removal. Air may also be trapped in the bed, also leading to the formation of channels and poor distribution of the liquid.
In addition, the size and cleanliness of the bed particles are extremely important to the success of the filtration process; a bed composed of large particles allows significant numbers of small particulates to pass through the filter bed along with the filtered fluid. On the other hand, beds composed of smaller particles can become clogged with extremely small particulates, rapidly rendering the filter bed ineffective. Sand also adsorbs organic compounds on which microorganisms can feed. The highest nutrient concentration is on the surface of the sand granules, so that is where the microorganisms grow. This microbial growth clogs filters and shortens the time interval until cleaning is required
Finally, large volumes of clean liquid are required to backwash and clean conventional filter beds, leading to large volumes of contaminated liquid which must be treated or properly disposed. Although backwashing is fairly effective for removing the particulates that are captured by the filter, some particulates may adhere so strongly to the bed particles that they are virtually impossible to remove, and the resulting loss in filter efficiency significantly impairs filter performance. Additionally, the specific gravity of the contaminant particulates is often equal to or greater than the specific gravity of the bed particles which make up the filter bed. In such circumstances, it is especially difficult to separate the heavy contaminant particles from the bed particles through a backwash process, and backwashing is therefore not effective as a cleaning method. In such situations, the contaminated bed must ultimately be replaced with new bed material.
In view of the above disadvantages with conventional filtration apparatuses and methods, it is the principal object of the present invention to overcome the above-discussed disadvantages associated with prior media fluid filtration systems.
Another object of the present invention is to provide anew and improved filtering system for the removal of particulate contaminants from process liquids which incorporates a high-efficiency backwashable filter bed.
A still further object of the invention is to provide a liquid filtration apparatus and method that embodies a filtration bed that floats on the liquid to be filtered.
Yet another object of the invention is to provide a liquid filtration apparatus and method that embodies a filtration bed formed from particles having a specific gravity substantially less than the liquid being filtered.
A still further object of the invention is the provision of a filtration apparatus and method that precipitates dissolved metals in a liquid process stream and then filters such precipitated metals from the liquid stream.
Yet another object of the invention is the provision of a liquid filtration apparatus and method that in one aspect incorporates a pair of filtration housings connected in parallel.
A still further object of the invention is the provision of a liquid filtration apparatus and method that in another aspect incorporates a pair of filtration housings connected in series.
The invention includes other objects and features of advantage, some of which, with the foregoing, will be apparent from the following description and the drawings. It is to be understood that the invention is not limited to the embodiments illustrated and described, since it may be embodied in various forms within the scope of the appended claims.
The invention disclosed herein overcomes the disadvantages encountered with prior bed filtration systems by providing a filtering apparatus which incorporates a bed medium with a specific gravity substantially lower than that of the liquid being filtered. During operation, the liquid to be filtered is withdrawn from a process tank or process stream and pumped under pressure through one or more filter housings containing the aforementioned bed medium. The liquid being filtered is pumped into the housing at the bottom, elevates the filter bed by floatation to a position where further elevation is restrained and then rises through the restrained bed medium, exiting the filter housing at the top.
A valve in the exit line at the top of the housing directs the filtered liquid either back into the process tank or stream, or into a clean fluid storage tank. A small storage tank in the filtration system provides a volume of filtered liquid for use in backwashing the filter bed medium to clean it by removing the particulates it strains from the process liquid. A valve in the entry line is closed when the backwash is performed to prevent backflow of contaminated liquid into the process stream. A second valve, in the exit drain line, is opened to direct the xe2x80x9cdirtyxe2x80x9d liquid into a storage chamber where it is collected until it can be properly disposed or recycled.
In one aspect thereof, the present invention is directed to an apparatus for filtering particulate contaminants from contaminated liquid process streams, such as, by way of example, from the cooling solution used during electrical discharge machining. The apparatus includes a primary pump with a liquid inlet from the process stream or process storage tank and a liquid outlet to the filter housing a chamber. The filter chamber includes a liquid inlet from the primary pump and a liquid outlet which returns the filtered liquid to the process stream or to a clean process fluid reservoir. The filter chamber contains the filter bed which acts as a strainer to remove the particulates from the process stream.
In another aspect, the present invention is directed to a liquid filtration apparatus that includes a filter bed composed of a particulate medium that has a substantially lower specific gravity than that of the process liquid to be filtered. The particle size and nature of this bed medium are determined by the identity of the process liquid to be filtered. As the process liquid is pumped through this filter bed, the particulate contaminants are strained from the liquid by one or both of the aforementioned methods.
In a third aspect thereof, the present invention is directed to a liquid filtration apparatus that includes a backwash system that incorporates a backwash reservoir to store cleaned process fluid for backwashing, a backwash pump, a single or plurality of backwash nozzles, a backwash valve, and a backwash waste liquid/particulate collection reservoir. During the backwash cycle, stored cleaned process liquid is withdrawn from the backwash reservoir by the backwash pump and forced through the spray nozzle(s). This backwash spray, in conjunction with alternately opening and closing of the backwash valve at the bottom of the filter chamber, serves to efficiently clean the strained particulates from the bed and wash them into the waste liquid/particulate collection reservoir.
The above-mentioned and other features and objects of the invention and the manner of obtaining them will become apparent and the invention will be better understood by reference to the following description of preferred embodiments of the invention taken in conjunction with the accompanying drawings.